In most types of agricultural, industrial and construction equipment, the speed of travel is directly a function of engine speed for any given drive train ratio. The operator of such a piece of equipment, such as a wheeled loader for example, determines the speed of travel by controlling engine speed. This is a convenient and effective means of speed control. The engine on such equipment also drives hydraulic pump or pumps to supply fluid pressure for manipulation of hydraulically-actuated equipment mounted on or towed behind the vehicle. The output of these pumps is related to their speed of rotation which is proportional to the speed of the engine driving them. Thus, pump flow will decrease as engine speed decreases. Engine speed is reduced by choice when the operator wants to slow the vehicle's speed and/or by load imposed on the engine when the operator is performing work with the equipment. It is most evident in the latter situation when the engine is lugged down by the combined load of tractive effort and hydraulic work, which is the product of flow and pressure. For example, in the case of a wheeled loader, the operator may fill the bucket by driving the vehicle forward with the bucket lowered in a digging attitude. As the tractive effort forces the bucket into the material; the boom arms supporting the bucket are raised and the bucket is rolled back. The combined load will, without the exercise of operator judgment and skill, kill the engine.
Some prior art arrangements on equipment with engine speeds which are operator-variable have provided a pump system in which a variable displacement pump is destroked as engine speed is reduced. However, this automatically reduces pump flow in not only those situations where engine speed is reduced by the load imposed, but also in those situations where engine speed has been reduced by operator choice simply to slow the vehicle's travel rate. As a consequence, pump flow, which is already reduced by lowered engine speed, is even further reduced by destroking one or more of the variable displacement pumps. The response of steering and/or hydraulically-actuated equipment is thereby slower.
Still other arrangements in the prior art have used a pressure compensator to destroke a variable pump when high system pressures occur. Again, there are situations where hydraulic response is slowed because of reduction in fluid flow when such reduction is not necessary to prevent killing the engine.
It is, therefore, an object of this invention to provide a hydraulic control circuit for a variable displacement pump which utilizes hydraulic logic to determine when conditions necessitate destroking of the pump.
It is also an object of the invention to provide a vehicle with an engine powering both a tractive effort drive train and a hydraulic work system with a means for utilizing the engine efficiently and to its fullest potential, and to accomplish this with only minimal input of operator judgment or skill.
It is another object of this invention to provide a hydraulic system which will permit responsive and rapid movement of hydraulic equipment under those conditions in which the engine is capable of supplying the necessary power and which will maximize that portion of the engine's power to the tractive effort drive train under those conditions in which the engine is incapable of meeting the power requirements of both the drive train and the hydraulic system.